Re: [PATCH for-6.18/block 04/10] blk-mq: convert to serialize updating nr_requests with update_nr_hwq_lock

[Nilay Shroff <nilay@linux.ibm.com>]

On 9/8/25 11:45 AM, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> request_queue->nr_requests can be changed by:
> 
> a) switching elevator by update nr_hw_queues
> b) switching elevator by elevator sysfs attribute
> c) configue queue sysfs attribute nr_requests
> 
> Current lock order is:
> 
> 1) update_nr_hwq_lock, case a,b
> 2) freeze_queue
> 3) elevator_lock, cas a,b,c
> 
> And update nr_requests is seriablized by elevator_lock() already,
> however, in the case c), we'll have to allocate new sched_tags if
> nr_requests grow, and do this with elevator_lock held and queue
> freezed has the risk of deadlock.
> 
> Hence use update_nr_hwq_lock instead, make it possible to allocate
> memory if tags grow, meanwhile also prevent nr_requests to be changed
> concurrently.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/blk-sysfs.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index f99519f7a820..7ea15bf68b4b 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -68,13 +68,14 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count)
>  	int ret, err;
>  	unsigned int memflags;
>  	struct request_queue *q = disk->queue;
> +	struct blk_mq_tag_set *set = q->tag_set;
>  
>  	ret = queue_var_store(&nr, page, count);
>  	if (ret < 0)
>  		return ret;
>  
> -	memflags = blk_mq_freeze_queue(q);
> -	mutex_lock(&q->elevator_lock);
> +	/* serialize updating nr_requests with switching elevator */
> +	down_write(&set->update_nr_hwq_lock);
>  
For serializing update of nr_requests with switching elevator,
we should use disable_elv_switch(). So with this change we 
don't need to acquire ->update_nr_hwq_lock in writer context 
while running blk_mq_update_nr_requests but instead it can run
acquiring ->update_nr_hwq_lock in the reader context.

So the code flow should be,

disable_elv_switch  => this would set QUEUE_FLAG_NO_ELV_SWITCH
...
down_read ->update_nr_hwq_lock
acquire ->freeze_lock
acquire ->elevator_lock;
...
...
release ->elevator_lock;
release ->freeze_lock

clear QUEUE_FLAG_NO_ELV_SWITCH
up_read ->update_nr_hwq_lock

Thanks,
--Nilay